At wavelengths in the deep ultraviolet range, that is, wavelengths less than 250 nm, mirrors having a positive refractive power are used in combination with lenses of negative refractive power as suitable means for color correction.
A catadioptric microscope objective having two concave mirrors facing each other is disclosed in Russian patent publication 124,665. The 60× magnification of the catadioptric microscope objective is achieved without intermediate imaging. Because of the low field size, only a few spherical lenses are needed for correction. A composite lens is used in addition to the mirrors for color correction. This correction means is, however, no longer available in the deep ultraviolet wavelength range.
Catadioptric objectives for microlithography having only one concave mirror are known from U.S. Pat. No. 5,691,802 or European patent publication 0,475,020. In these systems, the optical axis must be bent at least once. If reticle and wafer are to be mounted parallel to each other, then a two-fold beam deflection is required. This leads to significant complexity with respect to construction. If, in addition, a purely reflective beam splitter is used, such as disclosed in U.S. Pat. No. 5,691,802, then only off-axis object fields can be imaged. The lenses of the objective near to the field are non-symmetrically illuminated whereby asymmetrical thermal deformations and therefore imaging errors which are difficult to correct occur because of the absorption of the lenses. A centered arrangement of the optical components on a linear optical axis having two concave mirrors facing each other as shown in FIGS. 1 and 2 does not have this disadvantage. In contrast, an aperture obscuration occurs because of the cutouts in the mirrors.
The effects of an aperture obscuration on the contrast transmission function is investigated in the article of S. T. Yang et al entitled “Effect of Central Obscuration on Image Formation in Projection Lithography” (SPIE Volume 1264, Optical/Laser Microlithography III (1990), pages 477 to 485. For incoherent illumination, the contrast is reduced for low spatial frequencies in comparison to an unvignetted system. The acceptance of obscured objectives can therefore be significantly increased when the aperture obscuration is further reduced. In addition, a reduction of the contrast transmission function must not necessarily lead to a reduction of the resolution capacity because of the nonlinear response function of the photoresist. By suitably selecting the photoresist, the break in the contrast transfer function continues to lie above the exposure threshold of the photoresist.